Superconducting machines generally comprise superconducting coils which have to be reliably cooled, at least during the operation of the machine. Depending on the superconductor material used, different cooling temperatures are required, wherein materials with critical temperatures Tc of over 77 K have also been known for some considerable time. These materials are also referred to as high-Tc superconductor materials or HTS materials. Liquid nitrogen, liquid neon or especially liquid helium can be used as the coolant. This coolant is to be brought into contact with the winding or with its thermally-conductive support structure in order to cool the superconducting winding.
Because of mostly technical boundary conditions it is sometimes necessary, when cooling the winding, to transport the coolant against the force of gravity, thus to convey it from a lower position to a machine positioned higher up. This transport of the liquid coolant can be undertaken by way of mechanically-driven pumps, by way of bubble pumps or by an overpressure either applied from outside or created by evaporation of coolant. Mechanically-driven pumps are very maintenance-intensive, heat is generated during their operation, wherein the heat being introduced into the coolant must be avoided, and also the lifetime of such pumps is problematic. Bubble pumps avoid these disadvantages in part but are restricted in their pumping power through their function depending on the driving of a gas bubble in the coolant. The possible diameter of the riser pipe is restricted, as is also the rise height and the pump speed.
The transport of the coolant by way of overpressure is technically advantageous. For this purpose either controlled pumps and valves for the hot gas (in the event of the overpressure being applied from outside) or controllers, heaters etc are needed for the generation of the overpressure by evaporation. All these active components however conceal the risk of outages and in addition mean a not-insignificant additional outlay for the overall system.
A device which operates with separately-activated, time-modulated heating devices is known from DE 10 2010 041 194. In the device, a number of condenser chambers are each provided with one assigned cooling head, which are connected fluidically in each case via connecting lines with a joint evaporator chamber provided on the machine side. Each condenser chamber has a separate heating device. If coolant is to be conveyed from a condenser chamber via the connecting line to the evaporator chamber, the heating device of the respective condenser chamber involved is switched in, so that a temperature is set which is used for evaporating the coolant. This expands in the condenser chamber, resulting in an increase in pressure, through which the liquid coolant which is located in the connecting line to the evaporator chamber is pressurized in said chamber. The individual heating devices of the separate condenser chambers are switched on, modulated over time; they are thus operated successively offset in time to one another. The evaporation of the coolant thus occurs in the condenser chamber itself via the heating device assigned to the chamber.